Place parsley, watercress, capers, garlic, salt, and pepper in a mortar.
Crush with a pestle to make a smooth paste.
Add olive oil, 1 tablespoon at a time, beating vigorously with a fork or spoon after each addition.
Slowly add lemon juice, beating constantly.
Serve with artichokes, cooked spaghetti, shrimp, or any fried fish.